Process for making paper and the product thereof



Patented June 6, 1961 2,987,433 PROCESS FOR MAKING PAPER AND THE PRQDUCT THEREOF Kyle Ward, Jr., Appleton, Wis, and Jerrold A. Walecka, Westernport, Md, assignors to The Institute of Paper Chemistry, Appleton, Wis, a corporation of Wisconsin No Drawing. Filed Nov. 10, 1955, Ser. No. 546,274 4 Claims. (Cl. 162-157) The present invention relates generally to an improved method for treating pulp for the manufacture of paper and the product thereof, and more particularly, it relates to a method for chemically treating pulp to produce a paper having improved strength characteristics and other unique properties.

After pulp has been manufactured in any one of the several methods which are known to the paper art, it is generally further processed to develop wetness or reduce fieeness which is caused by the hydration of the cellulose fibers of the pulp. Freenes's or wetness is measured by the rate at which water drains from a stock suspension through a wire mesh screen.

Low degrees of freeness are generally induced by beating the pulp until it has a soft, slippery, slimy feel and a greater tendency to retain water than unbeaten pulp, though it will be understood that beating may be interrupted to provide pulp having intermediate properties. Beating is a well known process involving the mechanical treatment of papermaking materials suspended in water and may be accomplished in an apparatus known as a beater. There is a great deal of variation in the beater operations which is due to the particular type of equipment that may be used, as well as to the type of pulp that is being beaten. Because of these considerations, the heating time may vary from a short time to a period of as much as eight hours or more.

Longer periods of beating result in increased hydration of the pulp, but tend to weaken the strength of the resulting paper because of damage to the cellulose fibers in the pulp from the beater action. Since the beating not only hydrates the cellulose but also causes damage, the beating operation is desirably limited. In papermaking processes, it is, therefore, often necessary to adjust beating to reduce damage which results in reduced hydration. It would be highly desirable to increase hydration without increasing beating, and even more desirable to hydrate to a high degree with minimum beating.

In operation of the beater, blades are usually moved in close proximity to a cooperating bed plate, and consequently, beaters consume substantial power. Of course, a reduction in this power requirement in the manufacture of paper is preferred, provided however, that pulp having the same or increased hydration is produced as is provided with higher power utilization.

The main object of this invention, therefore, is to provide a method for treating papermaking materials to increase their sorption and thereby reduce the beating requirements in papermaking. It is also an object of the invention to provide an improved paper product. As will become more clear, hereinafter, the method of the invention involves treating cellulose fibers so as to introduce, to a low degree, hydrophilic groups comprising short chain organic radicals or comprising phosphate radicals, thereby preparing a cellulose pulp with improved hydrating and papermaking properties. Pulp which has been pretreated in this manner has less freeness than conventionally prepared pulps even before beating, and in the beater operation, the time and power required to achieve the desired degree of freeness is considerably reduced.

Paper sheets which have been produced from pulp treated in accordance with the process of this invention have had a tensile strength which is increased more than 60 percent and a burst strength which is more than percent greater than the strength of sheets made from untreated pulp. This increase in strength characteristics appears to result from the substitution of a short chain hydrophilic radical or phosphate radical, to a low degree, for hydroXyl groups on the cellulose molecule, which permits the pulp fibers to be more readily separated without breakage and also to more readily sorb water.

We have found that the degree to which the hydrophilic groups are added should be limited and, in this connection, excessive addition of hydrophilic groups results in solubilization of the cellulose with consequent loss in yield. In general, we have found that the degree of substitution should be less than .08, the degree of substitution (D.S.) being defined as follows:

mols of hydrophilic radicals substituted mols of cellulose Any lower degree of substitution provides improvement in the papermaking properties of the pulp but the degree of substitution is directly proportional to the burst strength of the resulting sheet and is inversely proportional to the freeness. Consequently, the desirable physical properties are reduced at lower degrees of substitution.

The hydrophilic groups which may be substituted in the cellulose include carboxyalkyl groups having carbon chains of less than four carbon atoms in the alkyl chain, hydroxyalkyl groups having less than four carbon atoms in the alkyl chain, alkyl nitriles having less than four carbon atoms in the alkyl chain, alkyl acid amides wherein the alkyl chain contains less than four carbon atoms, lower alkylesters of a lower alkyl acid, the alkyl groups of the ester and acid having less than four carbon atoms, and phosphate groups; and similar groups which provide ethers of cellulose and which have a hydrophilic character. In general, the longer chains in the alkyl groups result in reduced solubility of the cellulose so that the improved properties of the invention do not result.

In view of the foregoing, it will be seen that this invention is generally directed to the formation in pulp of ethers of cellulose, the ethers comprising only a small portion of the pulp and the alkyl chains of the etherifying compound generally having less than four carbon atoms in the alkyl chain.

It has been discovered that by this limited substitution of hydrophilic groups on the cellulose molecule the amount of water that is sorbed by the cellulose fiber is greatly increased, and consequently, the necessity for long periods of beating in the papermaking operation is substantially eliminated. The mechanism by which this result is accomplished is not completely understood, but it is evident that pulp which has been treated in accordance with this invention has greatly increased hydrophilic properties and may be formed into paper sheets which have increased tensile and bursting strength.

In general, the process of the invention involves the formation of a cellulose compound having the hydroxyl groups replaced, in low degree, with a radical which terminates in a hydrophilic group. This may be accomplished by carboxyalkylation, hydroxyalkylation, nitrilation, phosphorylation, oxidation, and like reactions. The reaction, in addition to being carried out in low degree, should be practiced in such a way as to prevent mercerization and, in this connection, the etherifying reactions may be carried out in the presence of alcohol solutions.

One method of preparing a substituted pulp of the type described is to substitute in the cellulose a carboxymethyl group for a few of the free hydroxyl groups of the cellulose molecule. This type of reaction may be obtained 3 by reacting monochloroacet-ic acid upon alkali cellulose. In this manner, some of the glucose residues of the cellulose molecule would form a modified compound having the formula:

Hydroxyethyl groups may be substituted on the cellulose molecule in place of the free hydroxyl groups of the glucose residues. This may be accomplished by reacting ethylene oxide with alkali cellulose.

As above indicated, the cellulose may also be treated with oxidizing agents, under controlled conditions, to form carboxy groups in place of some of the free hydroxyl groups of the glucose residues. The hydrophilic nature of the resulting pulp is shown by the fact that such pulp more readily sorbs water, as does the pulp substituted with the carboxymethyl groups and the hydroxyethyl groups.

Considerable care must be exercised in the treatment of pulp to limit the degree of substitution (D.S.) of the hydrophilic groups in the cellulose and, as pointed out, the D5. should not exceed .07. At .07 to .10 D.S., the cellulose fibers become partially soluble in water with loss in yield and no improved properties. In this connection, it should be noted that the improved sorbing qualities of the pulp are not achieved by the addition of such commercially available substances such as carboxymethylcellulose (CMC) to the pulp during the beater operation for the following reasons: (1) If CMC is merely added to the pulp, it is difficult to control the degree of retention of CMC in the pulp because of its solubility in Water; (2) The CMC which is sorbed on the pulp may be removed by washing; (3) The hydrophilic groups would be concentrated on random molecules in the mixture of materials and not distributed throughout the material; (4) The improvement in the strength characteristics brought about by the CMC decreases as heating continues while the strength characteristic of the partially substituted cellulose fiber of applicants invention is improved by the beating process.

A specific example of a method for replacing a few of the hydroxyl groups in cellulose with carboxymethyl groups is illustrated by the following example:

Carboxymethyl substituted pulps were prepared by solvent exchange drying of 400 grams of bleached rag halfstock using water, methyl alcohol and benzene for drying and then the pulp was air-dried from benzene. The pulp was divided into four equal portions, these being portions A, B, C, and D, and each of these portions was soaked for one-half hour in anhydrous isopropyl alcohol solution. Portion B contained .5 percent monochloroacetic acid, Portion C contained 1.5 percent monochloroacetic acid, and Portion D contained 5 percent monochloroacetic acid. Portion A contained no monochloroacetic acid. After the soaking period, the excess solution was filtered oil from each of the samples and the pulp transfered into 3.0 liters of a solution comprising 12.5 percent methyl alcohol, 87.5 percent isopropyl alco-' hol and enough dissolved sodium hydroxide to neutralize the monochloroacetic acid retained on the pulp and to provide an excess sodium hydroxide concentration of about 0.3 percent. The pulp suspensions in each portion were heated to a refluxing temperature on a stearn bath and maintained at the refluxing temperature for 45 minutes. The excess alcoholic solution was then decanted and the pulp neutralized with aqueous acetic acid. The pulp in each portion was washed several times with deionized Water and treated overnight with an aqueous solution of sodium bicarbonate to form the sodium salt of the ca rboxymethyl substituted cellulose fiber and finally the pulps'were washed repeatedly with deionized water.

Very little degradation'of the original pulp was ob tained even in the. sample which had been subjected to 5 percent monochloroacetic' acid. The degree of substitution of the hydroxyl groups in the cellulose for each'of thel samples' is given in the table below: 7 V

4 TABLE I Percent Portion Monochlo- D.S

roacetic Acid Low degree carboxymethyl substituted pulp which was prepared by the above procedure had several unique properties. As the degree of substitution was increased, the pulps became more slimy to the touch, more diificult to filter, more uniformly dispersed in suspension, slightly lower in freeness, and more highly sowellen in water than untreated pulps. All of these properties may be attributed to the more hydrophilic nature of the carboxymethyl substituted pulps and may be expected in carboxymethyl substituted pulps prepared by other methods or in cellulose pulps into which other hydrophilic groups have been substituted.

The series of carboxymethyl substituted pulps mentioned in Table I were treated in a Jokro mill to determine the beating properties of these pulps. Hand sheets (TAPPI) were made at various beater intervals and the strength properties of these sheets were tested. The Jokro mill runs revealed that the treated pulps could be beaten to a given freeness in less time than the untreated pulps. This is shown in Table II below:

7 TABLE 11 Initial Time to Time to Portion D.S Freeness, 600 cc. 400 cc.

cc. Freeness, Freencss,

min. min.

The strength properties of the hand sheets prepared from these pulps at the various beater intervals have been significantly increased, as shown in Table III, below:

TABLE III Maximum Maximum Portion D S. Burst Tensile Obtained, Obtained, pt./l00 lb. lb./in.

It is seen from these data that the bursting strength of the sheets formed from these pulps has been more than doubled and the tensile strength has been increased more than 60 percent over the strength of sheets from untreated pulp. This increase in strength is due to the hydrophilic nature of the carboxymethyl group substituted on the cellulose, and similar strength increases may occur when other hydrophilic groupsare substituted into cellulose or when the carboxymethyl group is substituted by a different method, in accordance with this invention.

In the foregoing method, bromoacetic and iodoacetic acid may be substituted for chloroacetic acid} Similarly, monochloropropionic. acid and monochlorobutyric acid may be employed in place of the monochlo'roacetic acid. In making these substitutions, the acids should be substituted in the molar equivalents to the monochloroacetic acid employed in the above described process. The various steps of the process may then be repeated.

In addition, to carboxyalkylation, the product of the invention may be prepared by utilizing methyl, ethyl and propyl nitriles and etherifying them with the cellulose. Likewise, acetic, propionic and butyric acid amies may be employed in etherifying the cellulose. Furthermore, ethers of cellulose with methyl, ethyl and propyl acetate, methyl, ethyl and propyl formate and methyl, ethyl and propyl butyrate may be produced.

As before indicated, phosphorylation and oxidation may be employed, in accordance with the invention. In this connection, phosphorylation may be effected by taking pulp which has been dried in the manner above described and immersing the pulp in concentrated phosphoric acid for a time sufiicient to effect a low degree of substitution. This may be accomplished by maintaining the pulp immersed until a loss of yield occurs.

Oxidation of pulp may be effected by taking pulp dried in accordance with the foregoing procedure and immersing it in alkaline potassium permanganate. The reaction is allowed to proceed to a D8. of less than .08 percent, whereupon the alkaline solution is drained from the pulp which is then washed free of the solution with deionized water.

It will be apparent from the foregoing that many procedures may be employed that result in the formation on cellulose of hydrophilic groups, in low degree, in place of the hydroxyl groups. As pointed out, the degree of substitution should be below the point where there is substantial loss of yield and, in this connection, is generally below .08.

Any papermaking materials which contain cellulose may be employed, in accordance with this invention. In addition to the rag stock above mentioned, linters may be used, as well as Douglas fir pulp.

Of course, pulp of the invention may be blended in various degrees with other pulp prior to beating or subsequent to beating. Various other ways of utilizing the invention will be apparent to those familiar with the art.

The various features of the invention which are believed to be new are set forth in the following claims.

We claim:

1. The method of making improved pulp which comprises replacing the hydroxyl groups of cellulose-containing pulp which has been subjected to a full alkaline digestion step to a degree of substitution less than .08 with hydrophilic radicals selected from the group consisting of phosphate, hydroxyalkyl and carboxyalkyl radicals wherein the alkyl chain has less than three carbon atoms, by reacting said cellulose under non-alkaline conditions with 50 a hydroxyl-replacing agent, whereby pulp having the fibrous character of paper pulp and having increased strength when fabricated into paper is obtained.

2. The method of making improved pulp which cornprises providing a cellulose-containing pulp which has 55 been subjected to a full digestion step, and replacing the hydroxyl groups in the cellulose to a degree of substitution less than .08 with hydrophilic radicals selected from the group consisting of phosphate, hydroxyalkyl and carboxyalkyl radicals wherein the alkyl chain has less than three carbon atoms, by reacting said cellulose under anhydrous conditions with a hydroxyl-replacing agent, and thereafter treating said cellulose to form an alkali salt thereof, whereby finished pulp having the fibrous character of paper pulp and having increased strength when fabricated into paper is obtained.

3. The method of making improved pulp which comprises providing a cellulose-containing pulp which has been subected to a full digestion step, and carboxyalkylating said cellulose to a degree of substitution less than .08 with hydrophilic carboxyalkyl radicals wherein the alkyl chain has less than three carbon atoms, by reacting said cellulose under anhydrous conditions with an acid carboxylalkylating reagent, and thereafter treating said cellulose to form an alkali salt thereof, whereby finished pulp having the fibrous character of paper pulp and having increased strength when fabricated into paper is obtained.

4. The method of making improved pulp which comprises providing a cellulose-containing pulp which has been subjected to a full digestion step, and carboxymethylating said cellulose, subsequent to all alkaline digestion of said pulp, to a degree of substitution less than .08 by reacting said cellulose in alcohol with acid selected from the group consisting of monochloroacetic, monobromoacetic and monoiodoacetic acids and mixtures thereof, and thereafter reacting said cellulose with a sodium saltforming agent, whereby the sodium salt thereof is formed, whereby finished pulp having the fibrous character of paper pulp and having increased strength when fabricated into paper is obtained.

References Cited in the file of this patent UNITED STATES PATENTS 1,787,542 Miles et a1. Ian. 6, 1931 2,038,679 Richter Apr. 28, 1936 2,137,343 Maxwell Nov. 22, 1938 2,160,107 Maxwell et a1. May 30, 1939 2,236,544 Maxwell Apr. 1, 1941 2,249,754 Ellsworth July 22, 1941 2,533,145 Schorger Dec. 5, 1950 2,626,214 Osborne Ian. 20, 1953 2,856,402 Turner Oct. 14, 1958 OTHER REFERENCES Daul et al.: "Studies on the Partial Carboxymethylation of Cotton, from Textile Research Journal, vol. 22, No. 12, December 1952, page: 787-792. 

1. THE METHOD OF MAKING IMPROVED PULP WHICH COMPRISES REPLACING THE HYDROXYL GROUPS OF CELLULOSE-CONTAINING PULP WHICH HAS BEEN SUBJECTED TO A FULL ALKALINE DIGESTION STEP TO A DEGREE OF SUBSTITUTION LESS THAN .08 WITH HYDROPHILIC RADICALS SELECTED FROM THE GROUP CONSISTING OF PHOSPHATE, HYDROXYALKYL AND CARBOXYALKYL RADICALS WHEREIN THE ALKYL CHAIN HAS LESS THAN THREE CARBON ATOMS, BY REACTING SAID CELLULOSE UNDER NON-ALKALINE CONDITIONS WITH A HYDROXYL-REPLACING AGENT, WHEREBY PULP HAVING THE FIBROUS CHARACTER OF PAPER PULP AND HAVING INCREASED STRENGTH WHEN FABRICATED INTO PAPER IS OBTAINED. 